No-ip and access to hd by ftp

I had a similar problem:

The firmware of the VMG8825-B50B modem is customized by Wind, but it was enough for me to understand that to answer me from the internet it was not my server from the PC but the modem that I found the solution. I changed the FTP port to enter the modem from 21 to 2121 and turn it on the internal LAN on port 21, at which point the server started working. I need the FTP client to remotely use port 2121 and thus coming in from the Internet the modem does not consider it an FTP service so it does not appropriate it but it changes it by changing the port and assigning it to port 21 and then making it FTP for my server.